The present invention relates to hydrostatic transmission apparatus for a vehicle having a front group of displacement members and a rear group of displacement members, the displacement member(s) of one of said groups being steerable members.
The apparatus comprises a transmission circuit which includes a pump, and hydraulic motors both in a front group of motors and in a rear group of motors, for driving the displacement members in the front and rear groups. A first one of the front and rear groups of motors comprises at least two motors in which two displacement members situated on either side of the vehicle are respectively coupled, while the second group of motors comprises at least one motor to which at least one displacement member is coupled, the two motors of the first group of motors being dual-capacity motors each having two distinct operating cylinder capacities which correspond respectively to a first elementary motor and to a second elementary motor, the first elementary motor of each of the dual-capacity motors being connected to the pump in parallel with a motor of the second group of motors, while the second elementary motor is connected in series with said motor of the second group, first and second series pipes thus existing between the second elementary motors of each of the two dual-capacity motors and the second group of motors.
Depending on the type of vehicle, provision may be made for the steerable displacement members, e.g. steerable wheels, to be either the rear displacement member(s) or else the front displacement member(s).
Similarly, the xe2x80x9cfirstxe2x80x9d group of motors, i.e. the group that always comprises at least two dual-capacity motors, may be the front group of motors or the rear group of motors. Depending on circumstances, the other group of motors may comprise one dual-capacity motor or two single motors, each of which drives a respective displacement member, or else two dual-capacity motors, each of which drives a respective displacement member.
Apparatus of that type is disclosed in Documents EP 0 547 947 and FR 2 719 001.
Connecting the second elementary motors in series in that way makes it possible to synchronize the wheels that are coupled to them with the wheel(s) coupled to the motors of the second group of motors. Connecting the first elementary motors in parallel in that way makes it possible to obtain torque values at the outlets of the motors that are equal.
During turning, the wheels on the outside of the turn must cover a distance longer than the distance to be covered by the inner wheels, and they must therefore be driven at a higher speed than said inner wheels. In practice, each of the wheels of the vehicle describes a curve whose radius of curvature is different from the radii of curvature of the curves described by the other wheels. For example, for a four-wheel-drive vehicle having two steerable wheels, the radius of curvature of the curve described by the steerable left wheel is different from the radius of curvature described by the non-steerable left wheel. The same applies to the right side of the vehicle.
Thus, for cornering behavior, the above-mentioned documents recommend momentarily interrupting the synchronization of the wheels by actuating valves which enable the series pipes to be connected momentarily to the pump, thereby making it possible to make the operation of the apparatus similar to that of a circuit in which all of the motors are disposed in parallel.
Unfortunately, with the apparatus of the above-mentioned documents, during turning, the two series pipes are connected via the same path to one of the ports of the pump. As a result, the pressure in both of the series pipes is substantially identical, so that the motors which are connected via those pipes all deliver substantially the same torque. Therefore, with apparatus of the above-mentioned type, when the valve for decoupling the motors during turning is activated, it is not possible to obtain a particular traction force on a given one of the wheels of the vehicle, even though it is necessary to do so. Furthermore, there is a risk of wheel spin in this situation.
Document EP 0 816 153 discloses other apparatus of the same type, in which valves make it possible to short-circuit momentarily the second elementary motors of each of the two motors of the first group of motors. When those valves are activated, only the motors disposed in parallel relative to one another remain active. That parallel configuration makes it possible, during turning, for the inner and outer wheels to be driven at different speeds. Unfortunately, it is not possible for the motors driving the inner and the outer wheels to deliver different torque values, so that it is not possible to generate different traction forces for the inner wheel and for the outer wheel.
An object of the present invention is to remedy the above-mentioned drawbacks so as to improve the behavior of the vehicle in particular during turning, by making it possible, during turning, for the inner and outer wheels to be driven at different speeds while being capable of delivering traction forces that are also different.
This object is achieved by the fact that the apparatus further comprises a selection valve connected to the first series pipe via a first connection, to the second series pipe via a second connection, and to a port of the pump via a third connection, by the fact that said valve is suitable for taking up a xe2x80x9cstraight-aheadxe2x80x9d configuration in which all three of the above-mentioned connections are isolated from one another, a xe2x80x9cleft-turnxe2x80x9d configuration in which the first and third connections are interconnected via a xe2x80x9cfirst left branchxe2x80x9d path, while the second and third connections are interconnected via a xe2x80x9csecond left branchxe2x80x9d path, and a xe2x80x9cright-turnxe2x80x9d configuration in which the first and third connections are interconnected via a xe2x80x9cfirst right branchxe2x80x9d path, while the second and third connections are interconnected via a xe2x80x9csecond right branchxe2x80x9d path, and by the fact that the flow sections of at least some of said right and left branches are different.
Thus, the left-turn and right-turn positions of the selection valve are specifically suited respectively for when the vehicle is turning left, and for when it is turning right. Since the flow sections in at least some of the right and left branches of the valve are different, head loss is generated in some of the branches relative to other branches, depending on the position of the valve. Such head loss means that the pressure at the downstream end of the branch is lower than the pressure at the upstream end, where upstream and downstream are considered relative to the fluid flow direction.
In particular, the flow sections in the first and second right branches are chosen to be different from each other, and the flow sections in the first and second left branches are also chosen to be different from each other. In this way, when the selection valve is in its left-turn position or in its right-turn position, the pressures in the two series pipes can be different, which makes it possible, while turning, to guarantee that the torques transferred, and thus the traction forces, are different between the various wheels of the vehicle, in particular between one or more inner wheels and one or more outer wheels.
For a vehicle of any given configuration, it is possible to define a steering diagram, i.e. to determine the curves described by each of the wheels of the vehicle in a turn of given curvature, which curves give the differences in the distances to be travelled by each of the wheels. Advantageously, the steering diagram is taken into account when determining the flow sections of the right and left branches of the selection valve.
Advantageously, the second right branch and the first left branch have substantially the same maximum flow section which is larger than the maximum flow sections of the first right branch and of the second left branch.
This constitutes a simple manner of setting the various flow sections of the selection valve for a vehicle whose behavior during right and left turns is substantially symmetrical.
Preferably, the apparatus further comprises control means for controlling the configuration of the selection valve, which means take account of the angular positions of the steerable displacement members.
The selection valve is a progressive valve, i.e. starting from its straight-ahead position, it goes progressively into one or other of its turn positions, for right turns or left turns, so as to open the right or left branches of the valves progressively.
The smaller the radius of curvature of the turn, the greater the extent to which the curves described by the outer wheels differ from the curves described by the inner wheels, so that the difference in speed necessary between the outer wheels and the inner wheels is greater.
It is thus advantageous to cause the valve to go progressively between its configurations as a function of the directions of the steerable displacement members. Generally, for a turn of very large radius of curvature, the valve can be displaced a little towards its left or right turn configuration so as to give rise to only a small difference in flow section between the first and second right or left branches. In contrast, in a turn having a small radius of curvature, the valve can be displaced to a much larger extent into its right or left turn configuration so as to give rise to a much larger difference in flow section between the first and second right or left branches.
The apparatus of the invention is applicable, for example, to a vehicle having four drivable wheels or having three drivable wheels, in which case the motor that serves to drive the isolated wheel, which is preferably the steerable wheel, is a dual-capacity motor, each of the elementary motors making up the dual-capacity motor being connected in series to the second elementary motors of respective ones of the two dual-capacity motors that drive the other two wheels.